Traditional oil production processes, often referred to as primary and secondary recovery, may only be capable of recovering 20-40% of the reserves in an oilfield reservoir. As such, a demand has arisen for more advanced processes capable of extracting additional reserves from existing oilfield reservoirs.
As an example, chemical enhanced oil recovery (EOR) is a technique that attempts to increase recovery factor of a field by injecting various chemical agents, including polymers (collectively referred to herein as EOR agents), into a reservoir. For example, an EOR technique referred to as polymer flooding injects water viscosified with soluble polymers into a reservoir to increase viscosity such that the mobility of the injectant is less than that of the oil phase in place. Doing so can improve oil-recovery sweep efficiency by creating a substantially smooth flood front. In addition, in heterogeneous reservoirs, the viscous injectant may flow along high-permeability layers to decrease the flow rates within these layers and enhance sweep of zones with lower permeabilities.
In order to improve the economics of such EOR processes, some EOR techniques inject an EOR agent into a reservoir in the form of a slug, which is then followed by chase water, such that the chase water effectively pushes the EOR agent through the reservoir, thereby reducing the amount of EOR agent needed to perform an EOR process.
In the case of polymer flooding, as well as for other EOR agents, the mobility of the chase water is generally higher than the mobility of the displaced slug, which can cause a phenomenon referred to as viscous fingering to occur in the contact area between the slug and the chase water. Viscous fingering can reduce the efficiency of a sweep and can cause significant volumes of recoverable oil to be bypassed. In addition, in severe cases, an early breakthrough of water into adjacent production wellbores may also occur.
Selection of an optimal EOR technique, or combination of EOR techniques, for a particular oilfield reservoir is highly dependent upon the properties of the reservoir (e.g., temperature, pressure, salinity, oil composition, rock properties, etc.) as well as additional concerns such as economic factors (e.g., up front capital investment, current and/or projected oil prices, ongoing implementation costs, etc.) Software-based tools have been developed to automate and otherwise facilitate the selection of EOR processes and generate estimations of incremental recovery that may be expected from EOR processes for a given oilfield reservoir.
For example, reservoir simulation may be used in some instances to model and design EOR processes. However, accurate capturing of complex physical and chemical processes may be required in order to obtain reliable field performance forecast results, and viscous fingering is amongst such processes. Simulation grid resolution is generally known to have significant impact on accuracy of simulation results, which holds particularly true for unstable displacement processes such as viscous fingering due to the relatively small dimensions of the viscous fingers. At the level of resolution needed to accurately simulate viscous fingering, however, the time and/or computational resources needed to run a simulation may be impractical for many situations. Inaccurate results can lead to, for example, poor EOR process performance, and in some cases, unexpected water breakthrough.
Upscaling may be used in some approaches to model viscous fingering in a less computationally-intensive manner. Such approaches, however, are generally limited to modeling processes where EOR agent is injected at a single, constant injection concentration. However, in many real world applications the injection concentration of EOR agent may be varied during field development, and conventional approaches can lead to inaccurate and/or erroneous results.
Therefore, a significant need exists for improved computer-implemented techniques for accurate and computationally efficient modeling of viscous fingering associated with an EOR agent during a reservoir simulation of an EOR process.